Electrophilic Aromatic Substitution Reactions

Transcription

1 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, 1 Electrophilic Aromatic Substitution Reactions An organic reaction in which an electrophile substitutes a hydrogen atom in an aromatic compound. *** Always think about RESONANCE *** Do I have resonance? How can I regain resonance? How does resonance influence the rate of this reaction? Aromaticity is VERY important and nice to have. Benzene rings are very stable, aromatic molecules. Therefore, the electrophile must be VERY strong in order to disrupt aromaticity. When doing EAS reactions, consider a few important things: How can I make my electrophile stronger? o Strong electrophiles REALLY need electrons, therefore, electrophiles are stronger when they are MORE POSITIVE (+, _+). Use a STRONG ACID to protonate a weak electrophile! How can I make my nucleophile (benzene ring) stronger? o Strong nucleophiles REALLY want to donate electrons, therefore, nucleophiles are stronger when they are MORE NEGATIVE (-, _-, lone pair electrons, pibond). Does your benzene ring have electron donating substituents with resonance? How can I make my reaction FASTER? o Reaction rate depends on the rate determining step (slowest step) o The rate determining step is the step which leads to the formation of the arenium ion (carbocation) o Therefore, you can make your reaction faster by increasing the stability of the arenium ion: resonance electron delocalization and/or better electron donating groups (substituents) on the benzene Two-step mechanism of all EAS reactions: (with adjustments, Thinkbook, P. 164 CFQ#7)

2 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, 2 1. ELECTROPHILIC ATTACK: Electrophile (+E) attacks the aromatic ring THE rate determining step: Requires a high energy of activation because aromaticity is lost. Leads to the formation of a resonance stabilized carbocation known as an arenium ion. (Hardinger Thinkbook, P. 161, CFQ#2) An Arenium Ion IS a carbocation. It has resonance! Think of the 3 Fates (Hardinger Textbook Chapter on EAS, P. 4) 1) Capture a nucleophile addition reaction; won t restore aromaticity. 2) Lose a proton, gain a pi-bond restores aromaticity! This occurs 3) Rearrangement won t accomplish much 2. DEPROTONATION: (Hardinger Thinkbook, P. 161, CFQ#2) The arenium ion is deprotonated by a weak base (:B) Requires a lower energy of activation because aromaticity is regained. * * * Two ways in which substituents donate or withdraw electrons: What makes the substituent UNIQUE compared to the other hydrogen atoms attached to the benzene ring? (resonance, higher electronegativity )

3 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, 3 Inductive Effect Electron Withdrawing If the substituent is more electronegative than the other hydrogen atoms attached to the benzene ring, the substituent has a greater electron withdrawing ability SLOWER REACTION Electron Donating If electrons on the sigma-bond of substituent move more readily towards the benzene ring, the substituent has a greater electron donating ability FASTER REACTION Resonance Effect delocalized lone pairs on the substituent can donate electron density to the benzene ring at the same time, the substituent is more electronegative than hydrogen and may inductively withdraw electrons (but has a lesser effect) Example: Hardinger Thinkbook Practice Problem, P. 178 #15a: (resonance contributors) *important resonance contributor: full octet for all atoms Characteristics of Electron-donating Substituents: lone pairs (except alkyl, aryl, and CH=CHR groups) resonance: delocalizes positive charge of a carbocation, which increases carbocation stability Characteristics of Electron-withdrawing Substituents: positive charge or partial positive charge high electronegativity ( electron greedy ) Substituent Directing Effect The electrophile attacks the nucleophilic pi-bond on the benzene ring, but the substituent on the benzene influences WHERE THE ELECTROPHILE ATTACHES in order to have the FASTEST RATE. Ortho/Para and Meta

4 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, 4 Important Qualities of Substituent (in order of importance): 1) MECHANISM: How do resonance and the degree of substitution on the arenium ion influence the reaction rate? RESONANCE: Increases the stability of the carbocation! There are always at least 3 resonance contributors for ortho, para, and meta: o Think of the resonance contributor where the positive formal charged carbon is directly attached to the substituent/director: o If the substituent is an electron donating substituent, it now can donate electrons to stabilize the electron-deficient carbocation. Resonance Contributors (Hardinger Thinkbook, Lecture Supplement P. 17) DEGREE OF SUBSTITUTION: increases the stability of the carbocation! Tertiary > Secondary > Primary Ortho, para > meta (Hardinger Thinkbook, P. 162CFQ #3) 2) Probability 2 ortho, 1 para, and 2 meta positions

6 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, 6 (amine) (ester) --OH --O--R (ether) --NR2 Halides: --Cl, --Br, --I (remember, they re also deactivators!) Influence on Rate? Which reaction is faster? < Has 4 resonance contributors. The most important resonance contributor has a full octet on all atoms: Increased stability Decreased activation energy Faster rate! Meta Directors: all substituents that withdraw electrons by resonance and/or inductive effects (EXCEPT halogens, which donate electrons by resonance, but withdraw electrons inductively) Common Meta Directors --NO2 --CF3 --+NR3 --C_N Why are electron withdrawing groups meta directors? If you have a substituent, which removes electron density from the benzene ring, you want to make the most stable carbocation possible. Compare the relative stability of resonance contributors for carbocations with a strongly electron withdrawing substituent like nitro (NO2):

7 Electrophilic Aromatic Substitution Reactions, Course Notes Archive, Ortho Meta Para Ortho and Para will form one resonance contributor that looks like this: This resonance contributor does not make a significant contribution to the stability of the arenium ion. In fact, the adjacent positively charged N destabilizes the carbon s open octet. Therefore, Ortho and Para positions have TWO resonance contributors, while the Meta position has THREE resonance contributors, none of which forms this destabilizing resonance contributor. Thus, you can conclude that meta directors are ortho-para inhibitors. Influence on Rate? Which reaction is faster? > Fewer resonance contributors (none of which has a full octet on all atoms) Less stable Increase activation energy Slower Rate! Regioselectivity or Major Product: Rule: the most stable mechanism intermediate will have the lowest energy of activation and the fastest rate, and therefore, be the major product. Why a substituent on the benzene directs an electrophile to a certain position, depends on the stability of the carbocation intermediate. Some Conditions Required in Order to Carry out a Reaction: Friedel-Crafts reactions won t happen with meta directors because the rings are too unreactive The order of a reaction is important in order to achieve the desired product. (Bruice, Organic Chemistry, P. 643) A STRONG ELECTROPHILE is necessary in order to overcome benzene s aromaticity Important EAS reactions:

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